"Then he realised it was a colossal skull, comprised of what looked like emerald stars, with a serpent protruding from its mouth like a tongue. As they watched, it rose higher and higher, blazing in a haze of greenish smoke, etched against the black sky like a new constellation."

Etymology

The incantation of this spell appears to be derived from the French mort, "death", and m00:30

The Dark Mark

Barty Crouch Jnr casts Morsmordre

ordre, "to bite". This alludes to the name of Voldemort's followers, the Death Eaters. The English word murder could also contribute. In Norwegian, Morsmordere directly translated means "mothers killers".

It may also be a reference to the word "mord", which means "murder" in Icelandic, German, Norwegian, Danish Polish and Swedish, since the spell is cast after someone has been murdered, and sounds like "mass murder".

In the book version of Harry Potter and the Half-Blood Prince, the Dark Mark was cast over the Astronomy Tower by Gibbon in order to lure Dumbledore there. However, in the film version, the Mark was non-verbally cast by Bellatrix Lestrange in celebration after Dumbledore was killed. Furthermore, in the film the Mark was dispelled by the mourning residents of Hogwarts as a sign of respect for the late headmaster.